A spin valve or a magnetoresistive (MR) sensor detects magnetic field signals through the resistance changes of a read element, fabricated of a magnetic material, as a function of the strength and direction of magnetic flux being sensed by the read element. The conventional MR sensor operates on the basis of the anisotropic magnetoresistive (AMR) effect in which a component of the read element resistance varies as the square of the cosine of the angle between the magnetization in the read element and the direction of sense current flow through the read element. Such a MR Sensor can be used to read data from a magnetic medium. An external magnetic field from the magnetic medium (the signal field) causes a change in the direction of magnetization in the read element, which in turn causes a change in resistance (ΔR/R) in the read element and a corresponding change in the sensed current or voltage.
A spin valve has been identified in which the resistance between two uncoupled ferromagnetic layers varies as the cosine of the angle between the magnetizations of the two layers and is independent of the direction of current flow.
An external magnetic field causes a variation in the relative orientation of the magnetization of neighboring ferromagnetic layers in a spin valve. This in turn causes a change in the spin-dependent scattering of conduction electrons and thus the electrical resistance of the spin valve. The resistance of the spin valve thus changes as the relative alignment of the magnetizations of the ferromagnetic layers changes.
Typically, a conventional simple spin valve comprises a ferromagnetic free layer, a spacer layer, and a single-layer pinned ferromagnetic layer, which is exchange-coupled with an anti-ferromagnetic (AF) layer. In an anti-parallel (AP) pinned spin valve, the single-layer pinned ferromagnetic layer is replaced by a laminated structure comprising at least two ferromagnetic pinned sublayers separated by one or more thin non-ferromagnetic anti-coupling sublayers.
In general, the larger the value of ΔR/R and the smaller the coupling field Hf, the better the performance of the spin value. The ΔR/R value of a spin valve conventionally increases as the thickness of the spacer layer decreases due to the reduced shunting of the sense current in the spacer layer of the spin valve. For example, a spin valve with a copper spacer layer having a thickness of 28 Å will achieve a ΔR/R of about 5%. If the thickness of copper spacer is reduced to 20 Å, a ΔR/R of 8% will be obtained. However, the ferromagnetic coupling field Hf also increases as the thickness of the spacer layer decreases. In addition, the ferromagnetic coupling field of conventional spin valves is unstable upon annealing cycles. For example, the ferromagnetic coupling field of spin valves changes from about +5 Oe at the beginning of the annealing process to +20 Oe after annealing cycles.
An article entitled “Oxygen as a Surfactant in the Growth of Giant Magnetoresistance Spin Valve” published Dec. 15, 1997 by Journal of Applied Physic to Egelhoff et al. discloses a method for increasing the giant magnetoresistance of ΔR/R of Co/Cu spin valves with use of oxygen. In this method, oxygen is introduced in an ultrahigh vacuum deposition chamber with an oxygen partial pressure of 5×10−9 Torr during deposition of the spin valve layers, or the top copper surface is exposed to the oxygen to achieve an oxygen coverage, after which growth of the sample is completed. The oxygen acts as a surfactant during film growth to suppress defects and to create a surface that scatters electrons more specularly. Oxygen coverage decreases the ferromagnetic coupling between magnetic layers, and decreases the sheet resistance of spin valves.
Unfortunately, this technique requires a very small oxygen partial pressure window around 5×10−9 Torr, since when the oxygen partial pressure is increased to only 10−8 Torr, all GMR (ΔR/R) gain due to oxygen is lost, and at oxygen pressures higher than this, the fall-off in GMR is rapid. This very small oxygen partial pressure is very difficult to achieve or to maintain in a large manufacturing type system. Also, oxygen exposure of only one surface of the copper spacer layer does not optimize the ferromagnetic coupling field. Furthermore, the use of oxygen for all spin valve layer depositions may result in oxidation of Mn in anti-ferromagnetic materials, such as FeMn, PtMn, IrMn, PdPtMn and NiMn, and thus kills the spin valve effect. Therefore this technique can not be applied for spin valve deposition.
In addition, adsorbing oxygen only on the copper surface does not improve the GMR, and produces only a positive coupling field. Furthermore, this technique results in a decrease in sheet resistance, which reduces the overall signal. Finally, prior art oxygen treatment does not show stabilization of the ferromagnetic coupling field upon hard bake annealing cycles.
There is a need, therefore, for an improved method of making spin valves that overcomes the above difficulties.